With regard to a vehicle including an engine and an automatic transmission capable of controlling engine output torque independently of accelerator pedal operation by a driver, there is a concept of “driving force control” that positive and negative target drive torque calculated based on a degree of pressing the accelerator pedal by a driver, a vehicle operation condition and the like is achieved as engine torque and a transmission gear ratio of the automatic transmission. Control schemes referred to as “driving force request type” and “driving force demand type” also belong to such a concept.
Japanese Patent Laying-Open No. 2002-87117 discloses a driving force control device capable of achieving driving force as requested by a driver and thereby significantly improving power performance and drivability, with such control specifications that a steady target and a transitional target of driving force are attained by tuned control of engine torque and gear ratio.
In a powertrain having an engine and a transmission, the driving force control device disclosed in this publication includes: accelerator pressing degree detection means for detecting a degree of pressing an accelerator; vehicle speed detection means for detecting a vehicle speed; target driving force operation means for operating static target driving force based on the detected degree of pressing the accelerator and vehicle speed; driving force pattern operation means for operating a pattern of variation in the target driving force; steady target value operation means for operating an engine torque steady target value based on the target driving force and operating a gear ratio steady target value based on the detected degree of pressing the accelerator and vehicle speed; transitional target value operation means for operating an engine torque transitional target value and a gear ratio transitional target value based on the pattern of variation in the target driving force; target engine torque achieving means for achieving the engine torque steady target value and the engine torque transitional target value; and target gear ratio achieving means for achieving the gear ratio steady target value and the gear ratio transitional target value.
According to the driving force control device, during running, the target driving force operation means operates the static target driving force based on the degree of pressing the accelerator detected by the accelerator pressing degree detection means and the vehicle speed detected by the vehicle speed detection means, and the driving force pattern operation means operates the pattern of variation in the target driving force. In addition, the steady target value operation means operates the engine torque steady target value based on the target driving force and operates the gear ratio steady target value based on the detected degree of pressing the accelerator and vehicle speed. The transitional target value operation means operates the engine torque transitional target value and the gear ratio transitional target value based on the pattern of variation in the target driving force. Then, the target engine torque achieving means achieves the engine torque steady target value and the engine torque transitional target value, and the target gear ratio achieving means achieves the gear ratio steady target value and the gear ratio transitional target value. Namely, control specifications are such that engine torque does not entirely compensate for generation of inertia torque involved with transmission delay of the transmission or variation in the revolution speed, but the steady target and the transitional target of driving force are achieved by tuned control of the engine torque and the gear ratio. Therefore, driving force as requested by the driver can be achieved, and power performance and drivability can significantly be improved.
In calculating the target driving force as driving force of a driveline on the drive wheel side relative to the transmission in driving force control, a gear ratio is required for operation of torque to be output from an internal combustion engine. In the driving force control device disclosed in Japanese Patent Laying-Open No. 2002-87117, an actual gear ratio calculated based on a ratio between input and output revolution speeds of the transmission is used.
If such an actual gear ratio is used, however, it is affected by fluctuation in the revolution speed or sensor accuracy, and control stability may lower. Meanwhile, it is also possible to use a gear ratio instruction value output from the control device controlling the transmission, however, deviation from the actual gear ratio may be produced in the event of failure, and in such a case, torque to be output by the internal combustion engine may not properly be calculated. In particular, if the torque to be output by the internal combustion engine is calculated by using the gear ratio instruction value while such a failure that a signal indicates a small gear ratio instruction value (2nd or 3rd on the high gear side) whereas the actual gear ratio is great (1st on the low gear side) occurs, the problem is noticeable. Here, the gear ratio is assumed as small, and torque to be output by the internal combustion engine is calculated as a great value. Consequently, relative to the torque to be output by the internal combustion engine that is calculated as a relatively large value, driving force actually output to the drive wheel side with a large actual gear ratio becomes excessively large.